Diesel particulate filters (DPFs) are widely employed as filters for trapping and removing a particulate matter contained in a soot-containing fluid such as exhaust gas discharged from a diesel engine. A DPF has an inflow end face through which exhaust gas enters, and an outflow end face through which exhaust gas is discharged, and has a honeycomb structure including a plurality of cells which are formed of porous partition walls having numerous pores (through holes), in which the inflow end faces of predetermined cells and the outflow end faces of the remaining cells are alternately sealed. When particulate matter-containing exhaust gas enters into a cell through its inflow end face, the gas flows through porous partition walls which form the cell and serve as filtration layers, and then the gas is discharged through an outflow end face. In this case, the particulate matter is trapped on the porous partition walls.
Generally, such a DPF is provided in the flow path of exhaust gas discharged from an engine. In such a case, physical properties of the DPF, which have no small effect on performance of the engine, must be measured in advance. Specifically, pressure loss in the DPF at a constant flow rate must be measured in advance as a part of specifications of the DPF.
Conventionally, filter pressure loss has generally been measured through the following procedure: a filter to be subjected to pressure loss measurement is provided in a predetermined fluid flow path; a fluid is caused to pass through the filter at a predetermined flow rate by fluid passing means such as a blower; and the fluid differential pressure generated when the fluid passes through the filter is measured (see, for example, Patent Document 1 and Non-Patent Document 1). In such a pressure loss measuring method (apparatus), the flow rate of a fluid has been regulated by, for example, controlling the rotation speed of a blower (i.e., fluid passing means) by means of an inverter. The apparatus described in Non-Patent Document 1 is basically employed for evaluating the tuning of an air supply/exhaust system of a vehicle engine or the like, and for performing relative comparison of pressure loss on the basis of the flow rate level with respect to a predetermined pressure loss value. Therefore, this apparatus cannot be employed for subjecting numerous DPFs to pressure loss measurement at a constant flow rate, thereby evaluating variations in DPF pressure loss.
Since the filter pressure loss to be measured slightly differs from filter to filter, when pressure loss is to be measured while the flow rate of a fluid is maintained constant, the rotation speed of a blower must be finely regulated for different filters. However, in the case where numerous filters are required to be subjected to pressure loss evaluation in mass production equipment, difficulty is encountered in finely regulating the rotation speed of a blower for different individual filters within a short period of time, and in obtaining reliable measurement results with few errors. In addition, difficulty in finely regulating the blower rotation speed requires an intricate measurement process, which poses a problem in that difficulty is encountered in subjecting numerous filters to pressure loss measurement.    Patent Document 1: Japanese Patent No. 2807370    Non-Patent Document 1: [online], SuperFlow Corporation, [Searched on Dec. 12, 2003], Internet <URL: http://www.superflow.com>